Tetrahydrofuran is obtained industrially on a large scale by cyclization of 1,4-butanediol (Weissermel, Arpe Industrielle Organische Chem., 4th Edition, VCH Verlagsgesellschaft Weinheim, 1994, page 111). Another possibility for preparing it comprises hydrogenation of dihydrofuran (EP-A 524 216).
The starting materials for the precursors in these cases are acetylene, propylene or propylene oxide, maleic anhydride or butadiene. Because butadiene is very readily available, there is now a preferential search for novel processes starting from butadiene and allowing tetrahydrofuran to be prepared in a simpler manner and at lower cost, the intention being in particular to reduce the number of reaction stages.
Intermediates based on the starting material butadiene are dialkoxybutenes which may be in the form of
1,4-dialkoxy-2-butenes (in cis and trans forms) and of PA1 1,4-dialkoxy-1-butenes.
They can be described by the general formulae EQU RO--CH.sub.2 --CH.dbd.CH--CH.sub.2 --OR I
and EQU RO--CH.sub.2 --CH.sub.2 --CH.dbd.CH--OR II,
where R is identical or different C.sub.1 -C.sub.15 -alkyl or cycloalkyl, C.sub.6 -C.sub.12 -aryl or C.sub.7 -C.sub.15 -aralkyl radicals.
The 1,4-butenediol diethers of the formulae I and II can be prepared in various ways, eg. by reacting dibromobutene with two equivalents of alcohols, by oxidative additions of alcohols onto butadiene (SU-A 1 046 238 or EP-A 462 031) or by addition of alcohols onto vinyloxirane (WO-A 8 902 883).
Another method, to which this invention likewise relates, comprises addition of alcohols onto butadiene to form monoalkoxybutenes as disclosed in WO 95/19334 and metathesis thereof to dialkoxy-2-butenes and 2-butene.